Do an extended ping to the computers from the switch, from the distribution and from the core.
Next, ask more questions: What applications are causing slowness. Did you check for line errors? How about speed/duplex settings?
See for yourself if what they are saying is true? Can you ask the two users to log in to a different machine and still get the same response?
When they are downloading emails, they are having slow network issues.
Ok. So do a continuous ping from the clients to the email server.
Check the interface(s) of the email servers for any line errors or line faults. Check the switch logs for any signs of interface bouncing.
Let it run and when they have the issue, stop it. Look for the time they had the issue and begin your investigation
View > Time Display format > Seconds since Previous Displayed Packet
Look for large gaps in time and look at the packets
Use the Expert Info view and look at Errors and Warnings tabs.